Answer:
If Chicago municipal bonds yield is 10% then Carter's treasurer make indifferent between the two.
Explanation:
Because Treasury Bond is exempt from tax income and both have same maturity, and they are equally risky and liquid; we then have the equation as below
Treasury bonds yield = Chicago municipal bonds yield after tax
⇔ 6% = Chicago municipal bonds yield * (1 - tax rate 40%)
⇔ 6% = Chicago municipal bonds yield * 0.6
⇒ Chicago municipal bonds yield = 6%/ 0.6 = 10%